It is proposed in U.S. patent application Ser. No. 236,063 filed Aug. 24, 1988 which names the present inventor as one of the joint inventors to combine a mechanical key and an electronic key in order to discourage theft of a motor vehicle. According to this proposal, matching of both a mechanical key and an electronic key is necessary to operate the vehicle, and an electronic key in the form of an infrared emitter is incorporated in the knob of a mechanical key. Matching of the electronic key takes place when the mechanical key is fitted into a key hole of a key switch and turned to a certain angle. Since the electronic key is much more difficult to duplicate than the mechanical key, an added security is thus obtained.
However, according to this proposal, should the electronic key become inoperable, for instance, due to a fault in the infrared emitter, it becomes impossible to operate the vehicle, and a serious inconvenience would be imposed upon the operator of the vehicle particularly because it would take some time to have the electronic key repaired.